Generally, a DC-DC converter is used to obtain an output direct voltage which is dependent on the turn ratio of a transformer for performing voltage conversion, and an output direct voltage from a DC-DC converter is often not compatible with a direct voltage which is required to be supplied to an electronic circuit.
In case an output direct voltage from a DC-DC converter is not compatible with a direct voltage requited by an electronic circuit, an output direct voltage from a DC-DC converter is supplied to a series regulator or a step-down chopper to be converted to a required direct voltage.
Such a series regulator is not expensive, however, if the difference between an output direct voltage from a DC-DC converter and a required direct voltage is large, there is raised a problem that loss of power is enlarged, which leads to low conversion efficiency. Consequently, a heat sink or a fan is required to be used so as to release radiated heat, which leads to difficulty in reducing a DC-DC converter in size.
On the other hand, a step-down converter may be used instead, in which loss of power is not large. However, since the number of oscillating switching units is increased, there is raised a problem that noise and interference (beat noise) with other outputs are undesirably induced. Furthermore, increase in the number of parts leads to a higher expense.
The applicant of the present invention suggests a DC-DC converter whose rectifying circuit has a reactor inserted therein as a DC-DC converter which can reduce loss of power in a comparatively simplified manner (Japanese Laying-Open Patent H11-18427).
On the other hand, in a conventional DC-DC converter, there is a problem that an output direct voltage cannot be set up freely by changing turn ratio of a transformer to be used.
In the case where a transformer 50 is used in a DC-DC converter, as shown in FIG. 1, a first output voltage V1 and a second output voltage V2 of the transformer 50 are proportional to a winding number, or a winding times, N1 at point “a” and a winding number N2 at point “b” of a secondary winding 50b of the transformer 50, respectively. That is, for example, if the first output voltage V1 is 12 V and the winding number N1 is 4, corresponding voltage for one winding is 3 V, thus the second output voltage V2 becomes (N2×3) V which is a multiple of 3 V. So, only an output direct voltage of multiple of 3 V can be obtained. So, in a conventional DC-DC converter, an output direct voltage of an arbitrary value cannot be obtained.